Idling Problem

LEWIS

1988 JAGUAR XJ12

V12

2WD

AUTOMATIC

157,000 MILES

Have a 1988 XJ12 jAGUAR, when starting cold at 1000 rpm and as it warms up will go as high as 1900rpm,
in manual it mentions could be auxillary valve, to test put in boiling water, did this but nothing happened, does the brass bulb on bottom suppose to move?